Fellner's public acting career began with her role on the TV movie, So-Called Friends (1997) in which she played the role of Glory Factor. Some of Fellner's multiple roles on television include her appearance on the cult comedy TV show, The Fast Show (2000), which gave her the opportunity to play various roles with Paul Whitehouse, Charlie Higson, Simon Day, and the rest of the cast, My Hero (2002), The Line of Beauty (2006), The Eustace Bros (2003), Hollyoaks (2006), where she was directed by her now-husband Adrian Vitoria, Casualty (2007–2009), and The Trip To Italy (2014) as Lucy, among others. Fellner has also appeared in many films including the role of Claire in Two Days, Nine Lives (2001] Boogie Woogie (2009) with Heather Graham and Gillian Anderson, Little Deaths (2010–2011) as Maria Joy/Elizabeth Gordon, and The Trip to Italy with Steve Coogan and Rob Brydon, and Two Jacks (2012) opposite Jack Huston and Sienna Miller Sienna Miller, among others. After the success of The Trip To Italy, which premiered at the Sundance Film Festival, Fellner then made a second movie with Michael Winterbottom called The Face of An Angel (2014), about the Amanda Knox trial, which should be released in the US in 2015.

Fellner has also appeared in multiple short films, including a modern-day adaptation of Hamlet, All Our Sins Remembered (1998) as Emily Brahinski, Incorporated (2010) as Clarice, and Hit Girls (2011) as Emma. Hit Girls was also her first film as a Producer.

Fellner is married to Director and partner at Rosebud Pictures, Adrian Vitoria. As a team, the couple has worked on several projects together, including the British heist movie, The Crew, which starred Fellner along with Stephen Graham and Rory McCann. Fellner and her husband barely escaped the 2004 Sri Lanka tsunami, which devastated the town of Hikkaduwa. Fellner candidly wrote about her experience in The Independent in 2005. Rosie Fellner has teamed up with the youth campaign Bite The Ballot. Since its establishment in April 2010, the project that began in a classroom has expanded into a national campaign with thousands of supporters promoting engagement of youth in politics.

Notably, Fellner has appeared opposite of Sean Bean in Age of Heroes (2012) and in Two Jacks (2013) with Jack Huston and Sienna Miller. Coming up, Fellner will appear in the movie 'Bus 657' (2015 – currently in post production) with Jeffrey Dean Morgan, Robert De Niro, and Kate Bosworth.